He said: “On 11 November, with Captain Walter Barrie, we had a major setback through the insider threat.

“I’m sure many people cannot understand how the very people we have come to help might turn their rifles on us, and it’s hard for us to come to terms with it.

“But the fact is that there are people out there who don’t want us to be here and we have to understand that and we have to endeavour to give ourselves the best protection.

“We look inwards and we look to find the insurgent within, and I have to say there are not that many.

“If there is a view that the ANA [Afghan National Army] is riddled with insurgents, we don’t have any evidence of that.”

He stressed such incidents might not always be connected to the insurgency, listing drug use and battle fatigue as alternative causes.

He also claimed that the Afghan troops being trained at Camp Shorobak, next door to 1 Scots’ Camp Tombstone, were just as wary of the insider threat as the British troops. “The reality is that such people would just as readily kill an Afghan soldier as an ISAF [International Security Assistance Force] soldier,” he added.

Col Wrench said Cpt Barrie’s death had been a devastating blow for ANA soldiers. He said: “They were in shock but they have been doing everything they can to support the investigation.”

2014 target

BRITISH front-line combat troops will be out of Afghanistan by the end of 2014 under the UK government’s plans to withdraw from the war zone.

Yesterday Prime Minister David Cameron chaired a meeting of the National Security Council to discuss the detailed timetable for withdrawing UK forces over the next two years.

It is understood that the plan is to keep the number of forces stable at 9,000 until September next year, and then withdraw 8,000 over the next 12 months, though details were not published.